Note: Descriptions are shown in the official language in which they were submitted.
~9~
20104-~1'27
The invention relates to a receiver including a
multipath transmlssion detector for cletecting multipath
interference in an angle-modulated input signal.
A multipath transmission detector suitable for use in
diversity receivers is disclosed in German "Offenlegungsschrift"
3,334,735. Receivers of such a type are, for exmple, in the form
of FM-receivers and are generally coupled to a plurality oi-
antennas. Depending on the multipath distortion of an input
signal incoming on an antenna, the receiver is switched by means
of the multipath transmission detector to one or more other
antennas receiving an input signal having a lower multipath
distortion~ Prior multipath transmission detectors include an AM-
detector and/or a frequency interference peak detector. In said
German Offenlegungsschrift multipath distortion is detected at the
simultaneous occurrence o~ sharp amplitude minima and frequency
interference peaks in the received input signal. Prior art
receivers including multipath transmission detectors have the
disadvantage that in many cases they detect multipath distortion
erroneously, in response to which the diversity system o~ such
receivers switches too often, which detrimentally affects the
sound quality of these receivers because of the fact that
generally audible phase jumps occur during the change-over
actions.
~ he invention has for its object to increase the
reliability with which multipath distortion is detected in such
receivers.
~.25910~
20104-8127
~ ccording to the invention, there is provided in a
receiver having a receiving frequency band for angle-modulated
signals in such frequency band, such receiver comprising a mixer
for deriving an I.F. signal from a received signal, a demodulator
for deriving a demodulated signal from such I.F. signal, and a
multipath transmission detector for detecting multipath distortion
of the received signal; the improvement characterized in that such
receiver further comprises a frequency window circuit haviny an
input coupled to either one of said mixer and said demodulator to
receive the signal derived thereby and an output connected to said
multipath detector, said frequency window circuit supplying at its
output an enabling signal to said multipa~h detector when the
derived signal at the input of said frequency window circuit
corresponds to a received signal within a frequency window having
a bandwidth narrower than and located in the receiving frequency
band of the receiver; such enabling signal enabling said multipath
detector to detect multipath distortion of the received signal.
An advantage is that distortions which are the result of
the narrow receiver bandwidth used especially in car radios, as a
result
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PHN 11.518 2 86.03.26
of which multipath distortion is erroneously ascertained in a manner to
be described, are no longer detected as multipath distortions. A further
advantage is that particularly in those systems which receive weak
signals with comparatively much noise the reliability with which
multipath distortion is detected can be significantly increased.
A preferred embodi~ent of the invention which includes a
demodulator is characterized in that the frequency window circuit is
connected to the demodulator and comprises a level detector, for
supplying an enable signal as long as the instantaneous amplitude of the
demodulated input signal is located within the values determined by the
level detector. It is an advantage that the level detector can be
realised in a simple manner and can be implemented in otherwise prior
art receivers.
The frequency window circuit may furthermoxe include a
low-pass filter between the demodulator and the level detector.
It is an advantage that because of the consequent
attenuation of high-frequency no.ise co~ponents in the demodulated in~ut
signal the sensitivity to noise during detectio~ of ~ultipath distortion
is decreased, ~hich still further improves the reliability of detection
of multipath distortion.
The invention further relates to a multipath transMission
detector including a frequency window circuit for use in a receiver
according to the invention.
The invention will now ~e described in greater detail by
way of example with reference to the accompanying drawing. Therein:
Figure 1 shows embodiments of the invention, and Figure
2a is a frequency curve representing the bandwidth of the receiver of
Figure 1;
Figure 2b is a frequency-versus-time curve of the
instantaneous frequency of an angle-~odulated input signal such as it is
received by the receiver of Figure 1;
Figure 2c shows an angle-~odulated signal produced by a
frequency window circuit shown in Figure 1, as a functisn of ti~e.
Figure 1 shows a receiver 1 ~hich is suitable for
receiving angle-modulated input signals such as phase or frequency
modulated signals. Receiver 1 may be susceptible to multipath
distortions caused by receiving from several directions signals
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PHN 11.518 3 86.03.26
reflected also by obstacles, such as ~ountains and buildings. In, for
example, FM-reception such reflected signals produce distortions in the
form of sharp amplitude ~inima and associated frequency interference
peaks, as i5 described in an article by ~.S.Corrington, entitled:
~Frequency-Modulation Distortion Caused by Multipath Transmission~,
published in Proceedings of the IRE, pages 87-891, December 1945.
As will be described hereinafter, multipath distortion is
very annoying, particalarly in car radios. Diversity receivers having a
plurality of antennas have been developed in order to counteract the
annoying multipath distortion. In such a diversity receiver a selection
is made, on the basis of the multipath distortion detected in the
received input signals,of little-distorted input signals originating
from one or more antennas, for which purpose a switch must be made from
one antenna to another. This switching action is generally audible and
1S must be limited.
The receiver 1 includes a diversity switch 3 having a
control input 2, and n inputs and one output, a unit 4 connected to the
switch 3 and comprising an HF stage and a mixer stage, an intermediate-
frequency amplifier unit 5 connected to this mixer stage and, connected
to the intermediate frequency unit , a demodulator 6 which is connected
to a loudsp~aker y~ , for example, a LF a~plifier, not shown The
diversity switch 3 is connected to n antennas 7-1 to 7-n. In addition,
the receiver 1 includes a multipath transmission detector 8 which on
detecting multipath distortion applies a control signal to the control
input 2 of the diversity switch 3. With the aid of this control signal
a switch is made to different combinations of antennas. In this manner
the receiver 1 always searches for an input signal of an appropriate
quality.
To this end, the multipath detector 8 may comprise an AM
detector 9. As in the angle-modulation principle the amplitude of an
angle-modulated input signal is constant, multipath distortion ~hich
causes the waves received by an antenna or an antenna combination to
quench each other by interference, in response to which ~harp amplitude
minima occur, can be detected with such an AM detector may comprise only
a frequency interierence peak detector 10 detecting by means of
amplitude detection frequencey interference peaks occuring in the
received signal, for example, the demodulated signal produced by the
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PHN 11.518 4 86.03.26
demodulator 6. As both phenomena, ~ the amplitude minima and the
frequency interference peaks can occur si~ultaneously in multipath
distortion, both an AM detector 9 and an inter~feren~e peak detector 10
can be used in the ~ultipath detector 8 and the detection results can be
simply combined with the aid of a weighting circuit 11 as shown .in
Figure 1 and known Per se from Ger~an Offenlegungsschrift 3,334,735.
Depending on the levels ofthe signals produced by the two detectors
9,10, the weighting circuit 11 applies, at the simultaneous occurence of
these two pheno~ena, a control signal to the control input 2 in response
to which the switch 3 changes-over. Particularly when used as a car
radio, the receiver 1 is a narrow-band receiver. This requirement is more
specifically dictated by the fact that the transmit frequencies of
trans~itters are close to each other. So as to increase the selectivity
of the receiver 1, the bandwidth of all the filters, such as the tuning
and inter~ediate fre~uency ilters in the units 4 and 5 of the
recei~er 1, must be comparatively narrow, a certain degree of distortion
because of this narrow-band nature results, however, in that a portion
of the side-band of the angle-modulated signal are not allowed to pass.
This causes the signal thus filtered thus not to have a constant
amplitude but rather to exhibit amplitude modulation. A further
consequence of the narrow-band nature is that in response to fast phase
changes in the amplitude-modulated signal after demodulation the
de~olated signal also contains what are com~only referred to as
frequency interference peaks. This has for its result that particularly
for highly ~odulated signals the multipath detector 8 detects an
amplitude mini~um and a frequency interference peak and then erroneously
produces a control signal which results in un~anted switching of the
diversity switch 3.
This disadvantage is counteracted by providing the
receiver 1 with a frequency window circuit 12 having a frequency window
the bandwidth of which is located of the receiver within the bandwidth
of the receiver 1 preferably sym~etrically around the central frequency
bandwidth. It has for its function to determine whether the instantaneous
fequency of the received, angle-~odulated input signal is within the
window bandwidth of the frequency window. For the time in ~hich this is
the case for a received signal, the frequency window circuit 12 applies
an enable signal to terminal 13 to indicate that any multipath
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PHN 11.518 5 86.03.26
distortion detected by the multipath detector 8 is sufficiently reliable,
as this distortion now is not the result of the fact that the receiver 1
is a narrow-band receiver. The terminal 13 is connected to the multipath
detector 8.
In a first embodiment the frequency windo~ circuit 12 is
then connected to the demodulator 6.
In the first embodiment the enable signal is generated as
long as the instantaneous fre~uency is located within the band~idth of
the frequency window circuit 12, and in the second embodiment as long as
the instantaneous amplitude of the demodulated input signal is located
within the levels of level detector 14 and consequently the
instantaneous frequency of the angle-modulated input signal is located
within the bandwidth of the fequency window ciruit 12. The bandwidth of
the frequency window circuit 12 is then, for example, approximately 50
15 kBz, this being located in the centre of the bandwidth of the receiver
1, which may amount to approximately 200 kH~. In such a case the levels
determined by the level detector 14 have been chosen such that they
correspond to a frequency window having a bandwidth of approximately 50
kHz.
Figure 2 shows graphs to illustrate the operation of the
frequency window circuit 12.
Figure 2a shows a frequency curve representing the
bandwidth of the receiver 1. Figure 2b shows a frequency-versus-time
curve of the instantaneous fre~uency of the angle-~odulated input
signal. The bandwidth having cut-off values f1- f2 of the frequency
window circuit 12 is located sym~etrically around the central frequency
fc and within the reciever bandwidth shown in Figure 2a. Figue 2c
shows an enable signal V13 as a function of til~e, which signal is
supplied from the terminal 13 and produced by the frequency window
circuit 12. In the areas I the instantaneous frequency of the input
signal is located between the cuf-off values f1l f2i accordingly,
an enable signal is produced in these areas I by the level detector 14.
This results in a frequency interference peaX :M due to multipath
distortion and an associated amplitude minimum, not shown, detected in
detector 9, the frequency interference peak occuring in an area I,
indeed being detectsd as a multipath distortion. In the areas II the
instantaneous frequency is located outside the frequency range from f1
PHN 11.518 6 86.03.26
to f2 of the fre~uency curve shown in Figure 2a. On the one hand, as
described already in the foregoing, this results in amplitude minima
when located at the edges of the curve shown in Figure 2a and in a
frequency interference peak because of an increase in noise caused by a
decrease of the signal-to-noise radio. The frequency intexference peak
detector 10 can easily be triggered by the noise increase, and the
amplitude detector 9 by an amplitude minimum. ~ut on the other hand the
absence of the enable signalin area II will inhibit triggering of the
multipath detector 8 Yi~ terminal 13; consequently distortion is not
erroneously detected as multipath distortion. It is an advantage that, in
those systems which receive weak signals with comparatively much noise
and where in areas II this noise adds to the said increase of the noise
due to the limited bandwidth of the receiver 1, the detection of
multipath distortion is significantly improved.
A still further i~provement of the multipath distortion
detection is acco~plished when the receiver 1 is provided with a
frequency window circult 12 which includes a low-pass filter 15,
connected to the demodulator 6, which filter is connected in turn to the
level detector 14. This cascade-arrange~ent has for its effect that the
frequency curve shown in Figure 2b is smoothed to some extent, whereby
the noise sensitivity on detection of multipath distortion decreases
still further and a further i~provement of the reliability with which
multipath distortion is determined is obtained. This is, fo~ example,
the case for fre~uency interference peak K. Because of the introduction
of the frequency window circuit 12 frequency interference peak K would
erroneously not be detected as multipath distortion, because this
frequency interference peak briefly exceeds the frequency limit f2.
The use of the low-pass filter 15 causes as is shown on a larger scale
in the inset, this frequenry interference peak to be partly smoothed as
illustrated by the broken-line curves, as a result of which the peak
stays within the frequency window so that multipath distrotion is
correctly determined.
From the above description it will be apparent that it is, if
wanted,possible to control in so~e degree instananeously the thresholds
of the level detectors in the AM detector 9 and in the frequency
interference peak detector 10, in dependence on the deviation in the
central frequency.
